quote:
Изначально написано samtakoi:
Спецы!? Ау! Ну проясните плиз ситуацию о взаимосвязи диафрагмы, дистанции фокусировки и резкости?
такая штука называется Shift-focus. Дальше гуглите =) статей не мало.
What is focus shift?
Focus shift is a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down.
Quite literally, the optimal plane of focus moves, depending on aperture! With every lens I've tested to date, the focus moves farther away. For example, if focus at f/1.4 is centered at 1.00 meters, then by f/2.8 it might now be centered at 1.02 meters. That apparently small difference means sparkling-sharp eyes versus not-quite-there eyes-it matters, especially with high-resolution digital cameras.
Focus shift is caused by spherical aberration (see vanwalree.com for an excellent technical discussion). Instead of a sharply-focused point of light a spherically aberrated lens produces a point of light with a "halo". This is visually confusing when focusing by eye (because of the lowered contrast) making it difficult to find optimal focus. It also is confusing to autofocus systems. In spite of these issues, accurate focus can generally be obtained-but it's no longer accurate when the lens is stopped down.
Это кстати не относится к браку тушки или объектива, это несовершенство оптической + механической части (так как камеры зачастую фокусируются на полностью открытой диафрагме)
http://www.the-digital-picture...ens-Review.aspx
для примера когда делали тест Zeiss 85 1.4 неавтофокусного (соответственно)
"Fast lenses of this optical design (without floating elements) shift the focus due to spherical aberration when the f-stop is changed. This phenomenon is especially visible on closer object distances and cannot be influenced. The AF system of most camera models does not respect those characteristics of a lens. The focus is measured and confirmed by the focus indicator as if the lens has been stopped down to f/5.6.
While using the lens wide open, the correct focus lies usually a little bit in front of the point that has been expected. Beside this, the focus confirmation function of any AF-camera is a relative improper tool for accurate focusing of an f/1.4 lens on closer distances. Also, a standard focusing screen and viewfinder of an SLR camera shows the limitations of accurate manual focusing with an 85mm lens at f/1.4. For instance, it is not possible in practical use to focus more accurately than about +/- 2 cm at an object distance of 1m without additional magnification systems. For accurate focusing at full aperture or stopped down a little bit, we strongly recommend:
-use a tripod if possible
-use zoom-in function in live-view mode while the lens has been stopped down to the designated f-stop.
-focus bracketing exposures (with small steps of rotating the focusing ring)"
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